Musnad Ahmad 1018:32

Yazid narrated to us, Hisham informed us, from Hafsa bint Sirin, from Abu al-Aliyah, from a man from the Ansar who said: I went out with my family intending to go to the Prophet, may Allah bless him and grant him peace, and there he was, standing, and there was a man facing him. I thought they had some business, so I sat down. By Allah, the Messenger of Allah, may Allah bless him and grant him peace, remained standing until I began to feel pity for him due to the length of the standing. Then he turned away, so I stood up to him and said: "O Messenger of Allah, this man kept you standing until I began to feel pity for you due to the length of the standing." He said: "Do you know who that was?" I said: "No." He said: "That was Gabriel, who kept advising me regarding the neighbor until I thought he would make him an heir. And had you greeted him, he would have returned your greeting."